This invention relates to a method for influencing the properties of cast iron by adding magnesium to the cast iron melt. The invention furthermore relates to a sensor for measuring the oxygen content in cast iron melts, which sensor contains an electrochemical measuring cell comprising a solid electrolyte tube.
Generally, the free magnesium content in a cast iron melt is considered as a determining factor for the formation of spheroidal or vermicular graphite in magnesium-treated cast iron. The current practice for regulating the production of ductile cast iron consists of determining the total magnesium content, i.e., the free and the bound magnesium, with the aid of spectrographically analyzed samples. However, this method provides an incomplete picture since the content of free magnesium is not known and the measurement does not provide any information about the oxygen activity. However, the activity of the oxygen, which is in equilibrium with the free magnesium, is a determining factor in the formation of the graphite form.
The so-called “ductile cast iron” is normal gray cast which is treated with a nodule-forming additive so that the main part of the graphitic carbon in the cast iron is nodular graphite or spherical graphite. Nodular graphite in cast iron must be analyzed in terms of form, size, and number of particles since these parameters influence the mechanical properties of the cast iron. A visual analysis is complex or subjective, even in partially automated analyses. Measurements in this respect are known, for example, from U.S. Pat. No. 5,675,097. German Patent Application Publication No. DE 19928456A1 describes measurements for the determination of the spatial structure of graphite in cast iron which are based on oxygen determination and do not have the disadvantages of visual methods. Thus, it can be performed faster, and the specific influence of the production increases the yield or, respectively, reduces the waste during casting. The quality of the cast iron can be well controlled.
The success of magnesium treatment in cast iron can be demonstrated, for example, by means of metallographic or spectrographic analyses of white solidified samples or by means of thermal analyses as well.
In general, pure magnesium or a magnesium alloy is used to promote the spherical form of the cast iron. One part of the added magnesium extracts oxygen and sulfur from the iron; the residual part is the so-called free magnesium which controls the oxygen activity. The free magnesium content in the melt is the determining factor for the nodularity of the cast iron. The free magnesium part decreases in the melt over the course of time while the oxygen activity increases. This influences the structure and the mechanical properties of the cast iron.
Sensors for the determination of the oxygen activity of a metal melt are known from German Patent Application Publication No. DE 10310387B3, for example. A solid electrolyte tube is disclosed which has, on its exterior surface, a coating of a mixture of calcium zirconate and a fluoride so that, for example, the concentration of sulfur, silicon, or carbon can be measured in iron melts.